1. Field of Invention
The present invention relates to a process for producing a fluorinated copolymer for an ion exchange membrane having carboxylic acid functional groups or sulfonic acid functional groups.
2. Discussion of Background
As an ion exchange membrane to be used for an alkali chloride electrolysis method in which an alkali chloride aqueous solution such as salt water is electrolyzed to produce an alkali hydroxide and chlorine, a membrane comprising a fluorinated copolymer having carboxylic acid functional groups or sulfonic acid functional groups has been known. The fluorinated copolymer is obtainable by copolymerizing a fluorinated monomer having a carboxylic acid functional group or a sulfonic acid functional group and a fluorinated olefin particularly tetrafluoroethylene (hereinafter referred to as TFE).
As a polymerization method, an emulsion polymerization method, a solution polymerization method, a suspension polymerization method or a bulk polymerization method may, for example, be mentioned. After the fluorinated copolymer is obtained by such a polymerization method, a step is required to separate and recover the polymerization medium or the unreacted monomer from the obtained fluorinated copolymer, and the following methods have been known. (1) A method of pouring a slurry obtained by solution polymerization to a poor solvent such as methanol to agglomerate the fluorinated copolymer (for example, Patent Document 1). (2) A method of purging the pressure of the reactor after polymerization, and evacuate the reactor of air by means of a cold trap with stirring to recover the polymerization medium and the unreacted monomer (Patent Document 2).
However, in the method (1), it is necessary to carry out agglomeration operation repeatedly several times and to distill off the solvent used for agglomeration to recover the fluorinated monomer, in order that the fluorinated monomer having a carboxylic acid functional group or a sulfonic acid functional group which is expensive, is completely recovered, thus leading to a high cost.
In the method (2), since an undecomposed initiator used for the polymerization is included in the recovered mixed liquid of the polymerization medium and the unreacted monomer, if the recovered polymerization medium and unreacted monomer are reused for the polymerization, a fluorinated copolymer having an unintended composition may form in the step of charging the monomer and the polymerization medium to a reactor and increasing the temperature and the pressure, whereby the current efficiency of the obtainable ion exchange membrane may be low. If distillation is conducted to remove the undecomposed initiator from the recovered mixed liquid, it will take long to recover the monomer and the productivity will be lowered.
Accordingly, a method has been desired in which the polymerization medium and the fluorinated monomer having a carboxylic acid functional group or a sulfonic acid functional group are recovered efficiently at a low cost without lowering the productivity, and are reused for polymerization without lowering the performance as an ion exchange membrane.    Patent Document 1: WO2009/133902    Patent Document 2: Japanese Patent No. 3,781,498